The inventive concepts described herein relate to a power amplifier circuit, and more particularly to a power amplifier circuit having improved efficiency and increased linearity by employing a pulsed dynamic load modulation technique.
The wireless communication industry is faced with an increasing demand for wireless communication systems that are capable of achieving a high data transfer rate with limited frequency resources. Accordingly, efforts have been made to optimize the modulation schemes used in wireless communication systems in order to meet the demand. Such optimization of modulation schemes has been devised with a view to achieving an increase in the Peak-to-Average Power Ratio (PAPR) of signals that are to be transmitted by the wireless communication systems.
Unfortunately, a power amplifier transmitting signals with high PAPR inevitably operates in a back-off power region more frequently than a power amplifier transmitting signals with low PAPR in order to maintain good linearity. However, frequent operation in the back-off power region may reduce an operating efficiency of a power amplifier. Accordingly, there has arisen a demand for a power amplifier that is capable of maintaining high efficiency even when operated in a back-off power region while maintaining good linearity.
One technique for avoiding a reduction in efficiency is disclosed in U.S. Pat. No. 8,254,854 entitled “Pulsed Load Modulation Amplifier and Method” issued to Wang et al. on Aug. 28, 2010, which is incorporated herein by reference. The disclosed technique avoids a reduction of efficiency by switching the power amplifier on and off by applying a pulse signal to a gate of the power amplifier. However, because the disclosed circuit uses a λ/4 line for impedance matching, the power amplifier tends to have a narrow bandwidth and is not operational for multi-band or multi-mode applications. Also, in order to recover an amplified original signal from an output modulated signal, the pulse signal for switching the power amplifier should have a frequency ten times higher than that of the modulation signal. This requires a switching frequency of about 50 to 200 MHz for Wideband Code Division Multiple Access (WCDMA) communication using a 3.84 MHz band or Long Term Evolution (LTE) communication using a 20 MHz band. However, since the input impedance of a power transistor in the power amplifier is large, a large current is required to switch the power amplifier on and off with such high frequency. Furthermore, switching noise may be induced during such on and off switching of the power amplifier, which may necessitate the use of a filter to remove the noise. Moreover, because the power amplifier has a narrow dynamic range, it may be difficult to perform precise control for low output power, thereby requiring an additional power control technique for low power levels, especially for the case of a WCDMA transmitter for example which requires precise control of output power as low as −50 dBm.
Accordingly, there has existed a need for a power amplifier circuit capable of overcoming the deficiencies of conventional power amplifiers employing a pulsed load modulation technique.